The advent of ultra-high-intensity lasers has opened up the possibility of producing high-quality electron, proton, X-ray and ion beams in facilities that are much smaller and less costly than a typical particle accelerator or synchrotron radiation source. The ability to generate intense electron and ion beams in particular could hold the key to the so-called fast-ignition approach to laser-driven thermonuclear fusion (M. Tabak, et al. Phys. Plasmas 1, 1626–1634; 1994), in which a target of hydrogen isotopes is first compressed by an array of laser beams, and then ignited by a single tightly focused higher-intensity beam that generates beams of fast electrons (or ions) within the resulting plasma. But when a laser is focused onto a simple planar target (with an intensity of around 1019 W cm−2 or higher), the MeV electrons produced emerge at a wide divergence angle of around 40°. This limits the ultimate intensity of the hotspot and is detrimental in most applications for which laser-driven beams are being developed.
The authors' results demonstrate that when a petawatt laser pulse interacts with a cone-wire target, the heating of the plasma is maximized close to the wire surface.Moreover, their simulations show that the complex field structures that emerge from this interaction (see figure) involves a reversal of the magnetic field inside the wire, which enhances the return current within a thin layer beneath its surface. This finding substantially improves our understanding of the guiding mechanism, and should enable further improvements in the design of cone-wire targets for a host of applications in medicine, materials science, physics and biology in which laser-driven electrons beams are expected to be used.
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